Documentation should take class extensions into consideration

As of right now, `deno doc` does not respect extensions. This means

```
export class B {
public id: number;
}
```

```
export class A extends B {
public userName: string;
}
```
if I run `deno doc` for class `A`, it will only document the property `userName` but **will not** for `id` part of the class extension.

`deno doc` should be able to read this, and respect the extensions of extensions (example, B extends C)

Start by running `deno doc` on the A and B class examples in the issue and inspect the documentation generator entry point that handles class members. Done means class A documents its own `userName` property, inherited `id`, and members inherited through further extensions such as B extending C.

Written by the indexing model from the issue text.